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PERCENT CAPACITY FIGURE 3 Pressure pulsations versus capacity

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PERCENT CAPACITY FIGURE 3 Pressure pulsations versus capacity pressure pulsations versus the frequency for selected flows. Figure 3 shows a typical plot of pressure pulsations versus capacity. As can be seen, a sudden increase in the magnitude of pressure pulsations indicates the onset of recirculation.

The onset of suction recirculation can be determined by an impact head tube (or pitot tube) installed at the impeller eye, as shown in Figure 4. With the tube directed into the eye, the reading in the normal pumping range is the suction head minus the velocity head at the eye. At the point of suction recirculation, however, the flow reversal from the eye impinges on the head tube with a rapid rise in the gage reading.

Corrective Procedures Every impeller design has specific recirculation characteristics. These characteristics are inherent in the design and cannot be changed without modifying the design. An analysis of the symptoms associated with recirculation should consider the following as possible corrective procedures:

1. Increase the output capacity of the pump.

2. Install a bypass between the discharge and the suction of the pump.

3. Substitute an improved material for the impeller that is more resistant to cavitation damage.

4. Modify the impeller design.

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